1. Field of the Invention
This invention relates generally to apparatus for converting solar energy to electricity by means of solar cells and more particularly to apparatus of this type which more efficiently collects and concentrates available sunlight for utilization by such cells.
2. Description of the Prior Art
It is well known that a photovoltaic semiconductor p-n junction can convert to electricity only that portion of the incident photon energy spectrum, typically solar radiation, which creates hole-electron pairs within a given semiconductor material. Thus, for example, in a silicon photocell only that portion of the solar spectrum with energy in the vicinity of 1.1 electron volts per photon is converted into electricity. Photons of lesser energy are not absorbed at all. More energetic photons are strongly absorbed and are wasted in heating the cell, which further degrades its energy conversion efficiency. Clearly, therefore, to maximize the efficiency of a given photovoltaic cell, it is advantageous to convert as much of the available light as possible into an energy range to which such cell can usefully respond before the light strikes the cell's surface.
An existing technique for achieving such conversion takes advantage of the fact that light falling upon a luminescent material or agent is characteristically re-radiated or emitted in a narrow band of wavelengths of known energy content. Also, light absorbed by such a material in one direction is scattered in many directions. Such agents include, for example, organic dyes which are used in scintillation counters, lasers and the like. For the purposes of this application the term "luminescent agent" is understood to include materials exhibiting all species of luminescence, including but not limited to fluorescence and phosphorescence.
It has been shown that a dispersal of such luminescent materials within an internally reflective sheet of transparent glass or plastic, one of whose major surfaces is exposed to the sun, concentrates and focuses a flux of light of known energy level toward one or more of the upstanding edge faces of such sheet. If a photovoltaic cell responsive only to light at that energy level is placed against or optically coupled to such edge face, the energy conversion efficiency of the cell increases several times. In this application a light-transmissive sheet of such construction and properties is termed a "luminescent sheet" and a photovoltaic solar collector employing such a sheet is termed a "luminescent solar collector". A luminescent solar collector of this type is fully and completely disclosed and applied in Optics, Vol. 15, No. 10, pp. 2290-2300, dated October 1976, the disclosure of which is incorporated herein by reference.
A thin, luminescent sheet of the type described produces a high multiplication of incident light intensity on an edge-mounted solar cell because of the relatively small area of an edge face in relation to the larger area exposed to the sun. For example, for a luminescent sheet 2 feet square and one-eight inch thick, this multiplication factor is approximately 200 to 1 if three edge faces are silvered. This light intensification theoretically enables one to increase solar cell power output per unit surface area by the same factor.
This greater light intensity may, however, result in correspondingly increased cell temperature either because of the relatively greater heating effect of high energy photons or as a result of IR heating under load conditions. If this temperature rises significantly, external cooling means may become necessary to avoid serious degradation of cell efficiency.
In addition, a certain amount of the available surface area of the cell facing the edge face of a luminescent sheet must be occupied by conductive elements for making connections between the cell and an external load circuit. Such conductors block the passage of useful radiation into the cell. The smaller the total cell surface area, the greater is the proportionate loss of total received light energy for conductors of a given size and current carrying capability.
Finally, as the edge face surface area becomes sufficently small, it is no longer feasible to employ edge-mounted solar cells of conventional design and size.
One solution to the potential problems outlined above might be simply to increase the entire thickness of the luminescent sheet. This has the drawback, however, of increasing the weight and cost per unit area of the sheet to a point which may become uneconomic.